A transparent conductive film is a transparent film through which a current can flow. The current main stream of the transparent conductive film is indium tin oxide (ITO). The ITO film exhibits excellent property, for example, a transmittance of 90% and a sheet resistance of approximately 10 Ω/square (also written as Ω/□), but the ITO film has problems of poor flexibility and resource depletion because indium is a rare metal. In addition, since the ITO film is produced by a vacuum deposition process, the cost is high.
Therefore, new electrode materials replacing ITO have been required. As such alternative materials, carbon nanotubes (hereinafter, referred to as “CNT”), metal nanowires, or conductive polymers are exemplified. Among these, CNT, which has an excellent electrical property and machine property, is expected as the most desirable material replacing ITO.
CNT has been attracting attention as a new material that exhibits various new functions, and active research and development are performed all over the world. In order to effectively use CNT, including the above-described transparent conductive film, for various industrial applications in the future, it is necessary to disperse multiple CNTs, instead of using one CNT alone, onto a substrate to form a thin film in which a network of CNTs is built.
Usually, when a dispersion solution of CNTs is dropped onto a substrate and is then dried, a CNTs network is built in a portion onto which the solution has been dropped, and then a CNT thin film is formed. However, the CNTs thin film exhibits properties different from a single CNT. For example, the conductivity of the CNTs thin film is significantly inferior to that of the single CNT. The reason for this is considered that electrical conduction between CNT and CNT greatly affects the conductivity of the CNTs thin film.
Accordingly, it is necessary to physically improve an electrical joint between CNT and CNT to build a high-quality CNTs network. For this, it is effective to prepare a node in a joint region.
In response to this, there is a known method in which, after forming a carboxylic acid group on the surface of the CNTs, a network structure of the CNTs is prepared using isocyanate as the node (Patent Document 1). Disadvantageously, in this method, an excellent electrical property of the CNT is degraded due to the formation of the carboxylic acid group on the surface of the CNTs, and an excellent electrical joint between CNTs is not formed because the isocyanate, which is a non-conductive material, is used for the joint.
There is a known method of preparing a CNTs network using a silica particle, by means of using a so-called alternate adsorption method of repeating a step of adsorbing a silica particle onto a substrate, electrostatically adsorbing the CNT thereonto, and further adsorbing a silica particle thereonto (Patent Document 2). Disadvantageously, an excellent electrical joint between the CNTs is not formed because the silica particle, which is a non-conductive material, is used for the joint.
It is suggested that the conductivity of the CNTs thin film be improved by preparing a dispersion liquid of conductive inorganic nanoparticles formed of a metal oxide and a CNTs dispersion liquid and alternately exposing these two dispersion liquids onto a substrate to distribute the inorganic nanoparticles throughout the CNTs network (Patent Document 3).
Disadvantageously, the conductivity of the CNTs network is not effectively affected because only an inorganic nanoparticle unintentionally disposed on a joint portion of CNTs affects the conductivity of the CNTs network, and the inorganic nanoparticle present on the surface of the CNTs do not contribute to the conductivity of the CNTs network. In fact, in example of this document, the sheet resistance of the prepared thin film is in a range of 1000 to 1500 Ω/square. It cannot be said that a film having a sheet resistance value in this range is enough to be used as a transparent conductive film.
It is suggested that the conductivity of the CNTs thin film be improved by bridging a metal disposed on a joint portion of the CNT in the CNT thin film (Patent Document 4 and Non-Patent Documents 1 and 2).
Disadvantageously, the transmittance of the film is decreased as the content of the metal particles is increased when the CNT transparent conductive film is prepared using the metal particles since the metal particles usually absorb visible light. Moreover, in order to bridge the metal disposed on the joint portion of CNT, the method described in Patent Document 4 is a complicated method in which a liquid containing metal ions is sprayed to the CNTs thin film and the current is flown in the CNTs thin film to conduct electrolytic plating of the metal to the joint portion of the CNTs, thereby depositing the metal. In order to flow the current, the CNTs network needs to be in contact with a metal electrode. Accordingly, this method is not practically used as a method of preparing a transparent conductive film that needs to be formed in a large area.
In regard to a transparent conductive film using CNTs, a method of preparing a uniform CNTs thin film and a post-treatment method for exhibiting conductivity are suggested by the present inventors (Patent Document 5 and Non-Patent Document 3). Disadvantageously, as a method of improving the conductivity of the CNTs thin film, a method of doping nitric acid, which is typically known, is used in place of a method of controlling a node of the CNTs network. Accordingly, it is difficult to stably maintain the conductivity for a long period of time.
As a dopant adhering to the outer circumferential surface of the CNT for the purpose of improving the electrical physical properties of the CNT, there is a suggestion of a donor having an ionization potential of 5.8 eV or less in a vacuum or a dopant material on which an acceptor having an electron affinity of 2.7 eV or greater in a vacuum is deposited (Patent Document 6).
Disadvantageously, since a principal of performing p type doping or n type doping is used for improving the conductivity of the CNTs thin film, the suggested dopant material cannot be used for a process of preparing a transparent conductive film in a mild environment, such as in the air, because the dopant material is extremely easily oxidized or easily reduced.
There is a suggestion of a method in which a conductive polymer such as polythiophene is used, as a dispersant, to the CNTs transparent conductive film, and Lewis acid, protonic acid, a transition metal halide, a noble metal halide, and an organic metal are doped as the p type dopant of the conductive polymer, or an alkali metal or an alkyl aluminum ion are doped as the n type dopant thereof (Patent Document 7). Disadvantageously, since the carrier mobility of a conductive polymer is inferior to the carrier mobility inherent in the CNT, a low sheet resistance is difficult to obtain even when the p type or n type doping is performed on the conductive polymer dispersant. Moreover, disadvantageously, since the doped conductive polymer strongly absorbs visible light, the absorption of the conductive polymer dispersant is a bottleneck to using as a transparent conductive film.